Use standard open-domain validation split in nq_open
The nq_open dataset originally drew the validation set from this file:
https://github.com/google-research-datasets/natural-questions/blob/master/nq_open/NQ-open.efficientqa.dev.1.1.sample.jsonl
However, that's the dev set used specifically and only for the efficientqa competition, and it's not the same dev set as is used in every open-domain question answering paper (including the Lee et al paper that introduced the open-domain variant of NQ, cited at the top of the dataset file). This PR changes nq_open to use the standard validation split and bumps the version to 2.0.0 since this is a breaking change.